FIGS. 3(a) to 3(g) show cross-sectional views for explaining a method of producing a primary diffraction grating according to the prior art which is disclosed in Opto-electronics session of 1987 Autoumn Meeting of Japanese Association of Applied Physics (Prescription No. 3 18P-ZR-14). FIGS. 4(a) and 4(b) are a plan view and a side view, respectively, showing resist patterning in the production process of the grating. In these figures, reference numeral 1 designates a substrate. Resist 2a is deposited on the substrate 1. A second time resist 2b is deposited on the substrate 1 after a secondary diffraction grating is produced thereon by etching using the first resist 2a as mask. Reference numerals 3a and 3b represent the periods of the secondary diffraction grating and the primary diffraction grating, respectively. Reference numeral 4 designates a sloped surface of the secondary diffraction grating produced by the first etching and reference numeral 5 designates an edge of the second resist mask pattern 2b.
The production process will be described.
First of all, a mask pattern of secondary diffraction grating comprising resist 2a deposited on the substrate 1 (FIG. 3(a)). The substrate 1 is etched using the resist 2a as a mask thereby to produce a secondary diffraction grating (FIG. 3(b)). Next, the resist 2a is removed (FIG. 3(c)) and second resist 2b is deposited covering the entire surface of the secondary diffraction grating (FIG. 3(d)). Next baking and development are carried out without light exposure thereby exposing convex portions of the secondary diffraction grating (FIG. 3(e)). Next, the substrate 1 is again etched from the exposed convex portions thereby to produce etched sloped surfaces, whereby a primary diffraction grating is produced (FIG. 3(e)). At last, the second resist 2b is removed (FIG. 3(g)).
Herein, the period 3b of the primary diffraction grating is one half of the period 3a of the secondary diffraction grating.
In this prior art method of producing a primary diffraction grating, the etched sloped surfaces 4 which are produced by the first etching in FIG. 3b) are likely to be concavo-convex reflecting the non-uniformity of the adhesion force of the resist 2a to the substrate 1. That is, at a position where the adhesion force of the resist 2a is strong, the etching depth is likely to be shallow and at a position where the adhesion force is weak, the etching depth is lilkely to be deep. Furthermore, the thickness of the second resist 2b which is deposited on the secondary diffraction grating is likely to be non-uniform reflecting the concavo-convex surfaces. Accordingly, when the convex portions are exposed as shown in FIG. 3(e), there is likely to be waviness at the pattern end 5 of the second resist mask 2b as shown in FIG. 4(a), and the primary diffraction grating which is produced utilizing the second resist mask pattern 2b having wavy end 5 is also likely to be irregular.